PURPOSE: To detect an illegal terminal, and to detect an address providing error at the time of set-up of a system, by prescribing the response timing of upstream data to the terminal concerned by an address duplication detecting command from the center.

CONSTITUTION: A random number generator 15 of a terminal 10 generates a random number in response to polling from the center 1, and a timer 16 is started. By control of this timer 16, the terminal concerned prescribes timing to be answered to polling from the center 1. Therefore, even if the same address has been provided to plural terminals, it becomes possible to make response timing to the center different. In this way, a collision of data on an upstream side is avoided, and the center 1 can detect the existence of duplication addresses.